The present invention relates to a free-fall grab, and more particularly to a free-fall, scissors-type grab for picking up ground samples from the sea bed.
A free-falling device for picking up ground samples from the bed of a waterway is known from German Auslegeschrift No. 1,911,782. This free-falling device includes two relatively pivotable grab halves arranged on the buoyancy member which halves are pretensioned by springs in the closing direction and are held in the open position by tension cables. For the purpose of closing the device, the tension cables are freed by a release mechanism which is effected dynamically by a percussion effect which is exerted by the buoyancy member on the release mechanism when the device strikes the sea bed. If this percussion effect is small because, for example, the surface of the sea bed is soft, no release will take place. Consequently, the halves of the grab are not closed and in addition, the weights which are resting on the grab halves also do not fall off so that the complete device remains on the sea bed. The danger of such a malfunction occurring is increased since the release mechanism is loaded by the total closing forces of the springs and the dynamic forces created by the impact of the weights, which are resting on the grab halves of the device, on the sea bed.
For avoiding non-release effects and hence, the loss of the complete device, it is already known to allow the free-falling device to sink without buoyancy members so that the device will strike the sea bed with its full weight. In this way, the reliable actuation of a switch is assured since the switch readily responds to a small force and an impact is not necessary to cause this small force, but just the weight of the device will suffice. The switch electrically fires a small explosive charge which actuates a piston for closing the halves of the grab and which in addition, forces water out of a cavity so that the necessary buoyancy is established. Because of the use of electric current and explosive charges, this arrangement is not only costly and complicated, but in addition, is unreliable. The explosive charge must be exactly measured and the switch must function reliably. Simultaneously, the current source must be sufficient. In the event of non-release, the complete device does not return to the surface and is, therefore, lost.
Our invention has for its object to provide a free-falling grab for picking up samples from the sea bed, which grab operates without springs that have to be tensioned for closing the grab halves, without electric current sources and without explosive charges. Moreover the grab of the present invention is simple and robust in design and construction and impact on the sea bed is unnecessary for the release of the grab since the grab is reliably released even when resting on the sea bed and thus its return to the surface is assured.
The object forming the basis of the invention is achieved by the fact that the grab is a scissors-type grab wherein the grab halves form the lower arms of the "scissor." When the grab is in the open position, the upper arms of the scissors frictionally and/or nonpositively engage the outer ends of a bar which is arranged on the buoyancy member in such a way that the bar holds the grab in an open position when the bar is not supported by the buoyancy member. The engagement between the upper arms and the bar is terminated when the bar is supported by the buoyancy member which is itself connected to the upper arms of the grab by means of cables or pivoted bars wuch that the grab is closed as a result of upward movement of the buoyancy member.
With the solution according to the invention, a reliable release of the closing mechanism is effected, even with a soft sea bed, i.e. without assistance of dynamic forces. During the descent, the upper arms of the grab are forcefully urged into engagement with the bar on the buoyancy member under the action of their own weight, and more particularly, of the depending, expendable weights mounted on the grab. When the opened scissors-like arrangement is resting on the sea bed, these forces are no longer exerted so the upper arms free the previously tensioned bar, and hence the buoyancy member is able to ascend. In principle, a frictional holding engagement between the upper arms and the bar is sufficient, but the bearing surfaces on the upper arms and the bar can also be of such an inclination that in addition, a positive force component becomes effective. Furthermore, it may be desirable to make this positive force component adjustable, by adjusting the slope of the bearing surfaces, or more particularly, by providing adjustable, threaded pins which are adapted to engage in corresponding complementary recesses.
According to another advantageous feature of the invention, the ends of the upper arms of the scissors-type grab are connected by a cable which, with the grab open, connects the ends of the upper arms over substantially the shortest path. The middle of this cable is connected to the buoyancy member. The use of a cable for closing the grab is a particularly simple and reliable means. It is further desirable for another or second cable to extend between the middle of the aforementioned cable and the buoyancy member. This additional cable is of such a length that the buoyancy member is operative or exerts an upward force on the grab only when its speed or ascent has been reached. In this way, not only the work, which can be produced by the buoyancy forces, is supplied for closing the grab, but simultaneously the kinetic energy of the upwardly moving buoyancy member is also used. Consequently, great reliability is provided with respect to the closing of the grab halves and more particularly, with respect to causing the weights to fall off of the grab.
In order to guarantee a reliable suspension of the grab on the bar of the buoyancy member, it is desirable for the bar to be pivotably connected to the buoyancy member. In this way, the grab is always depending perpendicularly beneath the bar so that the weights on the grab will not slip off the grab prior to the closing of the grab.
A further advantageous feature of the invention consists in the fact that the weights are arranged laterally on the grab. It is thereby assured that the weights will not interfere with the closing of the halves of the grab. Furthermore, this arrangement of the weights makes possible a very convenient release of the weights. The weights rest on a support which is provided on the grab and include, on their upper side, slots which are open upwardly and which are engaged by pins when the grab is in its open position. The pins are disposed on the upper arms of the grab and are disengaged from the slots with the closing of the grab. The weights are thus securely held on the grab during handling operations prior to descent and during the descent. Moreover, it is also possible, by proper design of the recesses and pins, to determine accurately at what closing angle of the grab halves the weights will fall off the grab. This means that it is possible to assure that the weights are disposed on the grab practically throughout the entire closing operation of the grab halves so as to press the grab firmly against the bottom and thus guarantee a suitable collection of material. On the other hand, it is nevertheless assured that the weights will be thrown off just before the closing movement is completed. All this is achieved with a very simple construction. The center of gravity of the weights is preferably chosen so that it is outside the support so as to further assure that the weights fall off of the grab.